unordered_map key无法取得时的的默认值

int main() {
    unordered_map<string, string> m1;
    unordered_map<string, bool> m2;
    unordered_map<string, int> m3;
    
    cout << (m1["a"] == "") << endl; // output 1
    cout << (m2["a"] == false) << endl; // output 1
    cout << (m3["a"] == 0) << endl; // output 1
}

output